Logger API

This package contains a logging API, such as Logger and Loggable interfaces along with some tools for their processing and very basic logger implementations.

The @run-z/log-z package is a reference implementation of this API. It supports structured logs, various logging mechanisms (e.g. log files), and customizable log formats.

Logger

Logger interface declares methods corresponding to generic logger levels:

• error(...args) - Logs error.
• warn(...args) - Logs warning.
• info(...args) - Logs informational message.
• debug(...args) - Logs debug message.
• trace(...args) - Logs tracing message. This may lead to outputting of stack trace.

Each method accepts arbitrary number of arguments.

Loggable Values

An object passed as an argument to one of the Logger methods can customize how it is logged by implementing a toLog() method of Loggable interface.

In the simple case, the toLog() method may return another value that will be logged instead of the original one.

It may also return an array of values that will be inserted to the log line instead of original value. This may be an empty array to completely remove the value from the log line.

A more advanced processing is possible by directly manipulating a DueLog parameter passed to the toLog() method. This parameter has the following properties that can be directly manipulated:

• on - A hint indicating the logging stage.

  A toLog() method may wish to conditionally process the message depending on the stage.

  Possible values are:

  • 'in' - input stage. Set for the logger input. I.e., for the log line passed to the logger method.
  • 'out' - output stage. Set by log writer. I.e., right before the message written to the log.
  • undefined - default stage. When set, the value should be processed unconditionally.

• line - Log line to process and log.

  Can be modified or replaced to change the message to log.

• index - An index of currently processed element of the log line.

  May be equal to the log line length to indicate additional value processing that may affect the message to log.

  Can be modified to specify the next element to process.

Every logger recognizes Loggable instances and processes them accordingly. To process the log line manually a dueLog() function can be used.

Tagged Log Line

The Logger methods allow to log any values. The arguments passed to one of these method called log line. It is up to the logger implementation of how to format the log line.

To customize the format of the log line a template string tagged by logline can be used:

import { consoleLogger, logline } from '@proc7ts/logger';

consoleLogger.error(logline`
  Request: ${request.method} ${request.path}
  Error: ${error.statusCode} (${error.statusText})
`);

// Request: GET /favicon.ico Error: 404 (Not Found)

The logline-tagged template formats the log line accordingly the following rules:

1. Template strings and values not separated by whitespace joined into single string.
2. The values separated by whitespace are added to the log line as is.
3. Template strings trimmed.
4. Any number of subsequent whitespace in template strings replaced with single space.
5. Leading and/or trailing template string removed if it became empty.

All Loggable values processed before being joined into string. They may be processed as many times as requested. The final joining happens at the output ('out') or default (undefined) logging stage.

Logger Implementations

Console Logger

consoleLogger is a Logger instance that logs to console.

Note that the first parameter isn't treated in any special way. I.e., it is not a format string.

Processing Logger

processingLogger() function creates a logger that processes Loggable values and logs with another logger.

Proxy Logger

proxyLogger() function creates a logger that proxies logging to another one.

The target logger can change dynamically.

Silent Logger

silentLogger is a Logger instance the never logs anything.